1. Field of the Invention
The invention relates to a variable valve timing apparatus for an internal combustion engine, which includes a hydraulic variable valve timing mechanism that varies the relative rotational phase of an output rotor with respect to an input rotor to change the valve timing.
2. Description of the Related Art
Conventional variable valve-timing apparatuses generally include, for example, a hydraulic variable valve timing mechanism, a hydraulic control mechanism and a controller, as described in Japanese Patent Application Publication No. 2009-203830 (JP-A-2009-203830). Hydraulic variable valve-timing mechanisms adjust the valve timing by varying the rotational phase of an output rotor with respect to an input rotor and lock the valve timing to the most retarded timing by engaging the input rotor with the output rotor.
The controller sets the duty ratio of the hydraulic control mechanism at 100% when there is a request to unlock the valve timing. Accordingly, the operation mode of the hydraulic control mechanism is changed to a state where the valve timing is advanced and is unlocked.
Thus, oil is supplied to advance chambers of the hydraulic variable valve timing mechanism, and drained from retard chambers. In addition, oil is supplied to a release chamber to withdraw a lock pin from an engaging hole. Accordingly, the engagement of the input rotor and the output rotor by the lock pin is released.
However, if the duty ratio is set at 100% when the valve timing is unlocked, the lock pin may become pressed against the engaging hole before the lock pin withdraws from the engaging hole to interfere with a withdrawal of the lock pin from the engaging hole.
Document EP 1 531 248 A1 discloses a variable valve timing apparatus for an engine, which has a hydraulic variable valve timing mechanism to change a valve timing by varying a rotational phase of an output rotor with respect to an input rotor and locks the valve timing to a specified timing by engaging input rotor with output rotor; a hydraulic control mechanism to control supply of hydraulic fluid to the hydraulic variable valve timing mechanism; and a controller to vary a duty ratio of the hydraulic control mechanism.
Document DE 100 55 770 A1 discloses a method for operating a valve for a hydraulic device. In the method, during unlocking, pressure is increased slowly with a lower oil pressure, and after the unlocking operation, an oil pressure control switches to a normal oil pressure control.
Document US 2003/0200944 A1 discloses a variable valve timing apparatus for an engine, which has a cam phase actuator, an oil pump, arithmetic means and a hydraulic pressure regulator. The arithmetic means determines a locked and unlocked state of a lock mechanism. An unlocking operation is performed at low rate.
Document US 2002/0166522 A1 discloses a variable valve timing apparatus for an engine, which has sensor means, cam shafts for intake or exhaust valves, an actuator driving the valves, a hydraulic pressure supply unit for the actuator, and a control means. The control means executes an unlocking operation of a locking mechanism.